1. Field of the Invention
The present invention relates to a Multiple Input Multiple Output (MIMO) wireless communication system. More particularly, the present invention relates to an apparatus and method for precoding by a midamble in a MIMO wireless communication system.
2. Description of the Related Art
A Multiple Input Multiple Output (MIMO) technique using multiple transmit/receive (TX/RX) antennas is being considered as a technique to satisfy an increasing demand for high-speed and high-quality data transmission. Since the MIMO technique performs communication using multiple streams via multiple antennas, it may greatly increase the channel capacity in comparison with the case of using a single antenna. For example, if a transmitter/receiver uses M number of TX/RX antennas, the channels of the antennas are independent of one another, the bandwidth and the total TX power are fixed, and the average channel capacity increases M times in comparison with the case of using a single antenna.
The MIMO technique is classified into a closed-loop MIMO technique and an open-loop MIMO technique according to whether channel information is fed back to a transmitter. Both the closed-loop MIMO technique and the open-loop MIMO technique are applicable to a Single-User (SU) MIMO technique that transmits a signal to only one receiver per unit of time. However, the closed-loop MIMO technique is also applicable to a Multi-User (MU) MIMO technique that transmits a signal to a plurality of receivers simultaneously. Thus, in a MU MIMO system, a transmitter obtains channel information for receivers.
The channel information may be fed back to the transmitter in various forms. Examples of the form of the channel information are quantized information of a channel coefficient estimated by the receiver and an index corresponding to an optimal code selected by the receiver from a codebook negotiated between the receiver and the transmitter. If the quantized channel coefficient is fed back to the transmitter, the channel information may be accurately represented so that the transmitter may perform a complex accurate MU MIMO technique. However, the feedback causes a large overhead. On the other hand, if the codebook index is fed back to the transmitter, the channel information is relatively inaccurate, but the feedback causes a small overhead.
If the codebook index is used, that is, if a codebook-based precoding technique is used, Mobile Stations (MSs) select suitable codes from the codebook and a Base Station (BS) generates a precoding matrix by using the codes selected by the MSs. For example, if a Discrete Fourier Transform (DFT) codebook is used, each of the MSs computes a correlation matrix of an estimated channel matrix and selects a code that is most similar to a dominant eigenvector of the correlation matrix. Thereafter, each of the MSs feeds back the channel information and the index of the selected code to the BS.
As described above, if the codebook-based precoding technique is used, feedback of the codebook index by the MS causes overhead and the code selecting operation requires an operation that computes the dominant eigenvector of the correlation matrix and compares the dominant eigenvector with the codes. Thus, there is a need to reduce computational complexity of the MS and the overhead caused by the codebook index feedback.